To create an electronic display screen, a micro-electromechanical systems (MEMS) based device such as a mirror is sandwiched between two glass plates: the back plate glass stand the substrate glass. The mirror is typically processed on the substrate glass. The back plate glass is then placed on top of the substrate glass to form the sandwich. The purpose of the back plate glass is to act as a viewing surface and to provide mechanical and environmental protection to the mirror. The sandwich is also referred to as the package.
The MEMS based device that is packaged in this manner is susceptible to problems associated with moisture and other harmful contaminants. The presence of moisture can cause stiction (static friction). The stiction can result because of the physical hydrogen bonding between the two glass surfaces in contact or because of the surface tension forces that result when the moisture between the two glass surfaces undergoes capillary condensation during the actuation of the MEMS based device. The presence of moisture can also cause electrochemical corrosion; for example, if the mirror includes an aluminum mirror.
The presence of harmful contaminants and moisture can pose a danger to the functioning of MEMS based device. For example, chlorine and moisture can combine to form an acidic environment that can be harmful to the MEMS based device. It is important that the package is moisture and contaminant free for the life of the device.
There are various channels by which water vapor or the contaminant can find its way inside the package. The moisture can enter the package from the environment in which the MEMS device is packaged. The moisture can permeate into the package from outside. The contaminant can be formed as a result of the outgassing of package components such as glass and polymers, especially at elevated temperatures.
In the prior art, to prevent the moisture and the contaminant from entering the package, the back plate glass and the substrate glass of the package are sealed to each other by using techniques such as welding and soldering, and by using o-rings. These prior art techniques are lacking in at least two respects. One, welding and soldering materials and o-rings occupy space. Real estate in MEMS based device packages is tight and there is a growing need for smaller form factors. Two, these prior art techniques do not eliminate the moisture and contaminants that are formed inside the package as a result of, for example, outgassing.
A simple technique to effectively seal two surfaces to each other that does not occupy additional real estate is desirable.